I think being a tech lead in a flat organization is more of a matter of being the techy that other techies want to talk to. Sure, those other people aren't your direct reports... but there are still plenty of leadership and mentor opportunities.
That's what is called "leadership without (formal) authority." If you are knowledgeable, helpful and approachable, your teammates will naturally gravitate toward you when they are in need of guidance. This sort of organic leadership is in many ways much more valuable than the artificial leadership provided by formal, top-down authority.
